The aim of a satellite radionavigation system is to determine the position of a receiver by using the principle of triangulation. The pseudo-distances measured on the basis of the signals received by three satellites are utilized to determine the position of the receiver.
In addition to the accuracy of positioning, two other parameters characterize the performance of a radio-navigation receiver: the capacity to operate with a low signal-to-noise ratio and the resilience to phase dynamics due to the motions of the carrier. In the case where the carrier is an aircraft, the accelerations of motion that it is liable to cause result in a significant Doppler frequency variation in the signals received by the receiver.
The means for demodulating the signals received by the receiver generally use the known phase-locked loop principle to accurately estimate the phase of the signals received so as to determine an accurate positioning. The performance of a phase-locked loop (PLL) is characterized notably by two criteria, its resilience to rapid variations in Doppler frequency and its capacity to operate at a low signal-to-noise ratio. A compromise is required between these two parameters, thus, the increase in resilience to Doppler frequency variations is achieved at the price of lower resistance to noise. Moreover, it requires an increase in the passband of the loop, thus giving rise to an overhead in terms of computational load for the receiver.
The resilience to dynamics of the known phase-locked loops is restricted by the phase discriminator which exhibits a limited linear span. The resilience to dynamics of a phase-locked loop represents its capacity to tolerate a significant change between two stationary states. In the case of slackness of the said loop, also called the carrier loop, during the transient phase between the two stationary states, the phase error may depart from the discriminator capture zone and this may give rise to the divergence of the loop and its dropout.
In order to increase the extent of the phase loop capture zone, it is known to implement a so-called “winder” discriminator which records the number of revolutions over the phase error, the consequence of which is that the capture zone becomes infinite in terms of phase error. It remains, however, limited in terms of frequency error or phase velocity error. This limitation is directly related to the sampling frequency used by the digital device which executes the loop. In a satellite radionavigation receiver, the limitation of the phase loop sampling frequency is related to the duration of coherent integration applied to demodulate the received signal.
This limit in terms of frequency may be penalizing in the case of applications, for example airborne, inducing large variations in phase velocity.
Another known solution is described in the applicant's French patent application No. 07 01931 which pertains to a device for receiving satellite signals comprising a phase loop with delay compensation. This solution makes it possible to increase the passband of the loop without rendering it unstable, the effect of which is to increase the resilience to swing of the loop without increasing the system pre-detection band, therefore without overburdening the computational load of the loops.
However, this solution, although making it possible to reduce the slackness of the phase loop and thus to improve the resilience to dynamics does not change the operating limit in terms of frequency of the discriminator which still constitutes the limiting factor.
Another known solution consists in making direct use of a frequency-locked loop but in this case, the phase measurement is much less accurate, this being penalizing to performance in terms of accuracy and integrity of the positioning measurement.